Comment by hnbad

> For whatever reason

The short answer is that your organs are mostly asymmetric and the esophagus connects to the stomach on the right so by laying on your right your stomach acid flows to the sphincter connecting your stomach and esophagus, making it easier to leak into the esophagus. This is especially a problem with GERD because you don't have pain receptors on that end of your esophagus but the stomach acid can still damage its tissue, which can lead to further complications including cancer.

A common cause of acid reflux is a herniated diaphragm (hiatal hernia) in which part of the stomach slips through the diaphragm into the upper chest. In this the esophageal sphincter will not be the highest point of your stomach, further adding to the problem when sleeping on your right side. Imagine a plastic water bottle deformed in such a way it stands slanted with the opening to the right, now imagine laying it on that side or the other. Laying it on the side with the opening makes it more likely to spill if the lid isn't forming a perfect seal.